1. Field of the Invention
The present invention relates to a transmit power control method and system employing a so-called double closed-loop control, which is suitably used in radio communications between a transmitting station and a receiving station.
2. Description of Related Art
Conventionally, transmit power control techniques have been employed in mobile communications. For example, International Publication WO97/50197 discloses double closed-loop control for regulating the transmit power of a transmitting station (either the base station or a mobile terminal) during radio communications between the base station and the mobile terminal in a CDMA mobile communications system. Double closed-loop control consists of inner-loop control and outer-loop control. Under inner-loop control, the transmit power of the transmitting station is regulated so that the SIR (Signal to Interference plus noise power Ratio) measured at the receiving station approaches a target SIR. During this process, data quality (frame error rate (FER) or bit error rata (BER), for example) of the received information is monitored at the receiving station, and the target SIR itself is updated so that the received data quality approaches the target quality (that is, outer-loop control).
With this double closed-loop control, transmit power control is performed efficiently during continuous data transmission between the transmitting station and the receiving station, while the signal quality received at the receiving station is maintained at a desired level (or the target quality).
However, if discontinuous transmission is performed between the transmitting station and the receiving station (for example, in burst mode, packet transmission, or transmitting control signals), problems arise in the conventional double closed-loop control.
FIG. 1 shows an example of such problems, illustrating the relation between the discontinuous data transmission and the target SIR in the conventional transmit power control. In FIG. 1, data transmission from a transmitting station to a receiving station terminates at time te, and resumes at time ts. During the idle period between data transmissions, the environment for radio wave propagation between the transmitting station and the receiving station is likely to change due to traveling of a mobile terminal (which may be the transmitting station or the receiving station). Such environmental change in radio communications includes, for example, fading and change in multi-path conditions between the transmitting station and the receiving station. Due to the change in radio wave propagation between the transmitting station and the receiving station during the idle period, the data receiving quality at time ts, at which data transmission is resumed, may deteriorate as compared with the data receiving quality at time te, at which the previous transmission terminates.
Under this situation, if the target SIR at the end of the previous transmission (at time te) is maintained until the beginning of the next transmission (time ts), that target SIR may be much lower than the correct target SIR (indicated by the dashed line in FIG. 1) that should be obtained based on the data receiving quality expected from the current environment of radio wave propagation in the idle period. For this reason, when the data transmission is resumed at ts, it takes time for the target SIR to converge to the correct level obtained by the outer-loop control. During the time delay required for the convergence, the data receiving quality inevitably deteriorates.